Funds are requested for purchase of a Meridian Instruments, Inc. ACAS 470 Interactive Laser Cytometer. This instrument comprises an integrated system for interactive study of rapid biological processes at the single cell level. The ACAS 470 is the only commercially available instrument in which a laser is used both to monitor and to interact with individual cells and subcellular regions in samples mounted on the stage of an inverted fluorescence microscope. Other features of the instrument include: an X-Y scanning microscope stage capable of positioning the sample with micrometer spatial resolution; an acousto-optic modulator for production of laser pulses on the millisecond time scale; a sensitive photomultiplier tube-based detection system; computer interfacing for all of the major instrument components; and computer software that controls the running and analysis of experiments. The ACAS 470 can be used for quantitative fluorescence microscopy and image analysis using single or double fluorescence labeling techniques. More uniquely, it can be used to quantify membrane protein and lipid lateral mobility, cytoplasmic tracer diffusion, cytoplasmic free Ca2+ concentration, cytoplasmic pH, membrane potential, and gap-junction mediated cell- cell communication. The laser can also be used to select individual cells of interest from a mixed population of cells. Nine major user groups, representing 5 departments and 5 institutions in the Harvard Medical Area, have been identified for use of this instrument: these groups share an interest in the molecular mechanisms of cell-cell interactions and of receptor- mediated signal transduction. Projects described by the major users encompass research on such mechanisms in human red blood cells, neutrophils, eosinophils, monocytes, platelets, and lymphocytes, and on endothelial cells, endocrine cells, nerve cells, and schistosomula of Schistosoma mansoni in culture. Acquisition of the ACAS 470 Interactive Laser Cytometer would provide a unique opportunity to extend previous studies of the major and other user groups to the forefront of interactive single cell research.